1. Technical Field
This invention relates generally to ignitors used for igniting air/fuel mixtures in automotive application and the like.
2. Related Art
Conventional spark plugs generally utilize a ceramic insulator which is partially disposed within a metal shell and extends axially toward a terminal end. A conductive terminal is disposed within a central bore at the terminal end, where the conductive terminal is part of a center electrode assembly disposed within the central bore. At the opposite/spark forming end, the center electrode is disposed within the insulator and has an exposed sparking surface which together with a ground electrode disposed on the shell defines a spark gap. Many different insulator configurations are used to accommodate a wide variety of terminal, shell and electrode configurations.
U.S. Pat. No. 6,883,507 discloses an ignitor for use in a corona discharge air/fuel ignition system. FIG. 1 is a diagram of a corona discharge ignition system according to the prior art. A feed-through insulator surrounds an electrode 13 as it passes through a cylinder head 19 into the combustion chamber 25. The insulator is fixed in an electrode housing 11 which may be a metal cylinder. A space 15 between the electrode housing 11 and the electrode 13 may be filled with a dielectric gas or compressed air. Control electronics and primary coil unit 7, secondary coil unit 9, electrode housing 11, electrode 13 and feed-through insulator together form an ignitor 5 which may be inserted into space 17. Ignitor 5 can be threaded into the cylinder head 19 during operation.
U.S. Publication No. 20100187999 discloses a device including two plasma generation electrodes, a series resonator and an induction coil surrounded by a screen. FIG. 2 illustrates components of a corona discharge combustion system according to the prior art. A spark plug used in a plasma generating system in accordance with the prior art. Spark plug 29 may be fixed to the cylinder head 39 of an internal combustion engine 105 of a motor vehicle. The spark plug 29 comprises a low-voltage cylindrical electrode which acts as a metal shell 37 intended to be screwed into a recess made in the cylinder head of an engine and which opens to the inside of the combustion chamber. An electrode is insulated from the shell 37 by an insulating sleeve. The insulating sleeve is made of a material the relative permittivity of which is greater than 1, for example a ceramic. The spark plug has a gap 41 separating the dielectric 100 from one end of the electrode 37.
The spark plug 29 also includes a shield 31 connected to ground and surrounding an inductive coil 33. The field lines are thus closed on themselves inside the shield 31. The shield 31 thus reduces the parasitic electromagnetic emissions of the spark plug 29. The coil 33 can actually generate intense electromagnetic fields with the radiofrequency excitation that is intended to be applied between the electrodes. These fields may, in particular, disrupt systems carried on board a vehicle or exceed the threshold levels defined in emission standards. The shield 31 is preferably made of a non-ferrous metal with high conductivity, such as copper or silver. In particular it is possible to use a conductive loop as a shield 31. The coil 33 and the shield 31 are preferably separated by an insulating sleeve 35 made of a suitable dielectric material, with a dielectric coefficient greater than 1, and preferably a good dielectric strength in order further to reduce the risk of breakdown or corona discharge, which cause energy to be dissipated. Provision may be made for the exterior surface of the sleeve 35 to be metalized in order to form the aforementioned shield 31.